The High Court has unanimously upheld the Federal Government's constitutional right to send asylum seekers to Papua New Guinea.

The court made the ruling after a challenge by lawyers for an Iranian man who arrived at Christmas Island in July last year and was transferred to the Manus Island detention centre.

Lawyers questioned sections of the Migration Act under which the Government is able to declare another nation as a "regional processing country" to which asylum seekers can be sent.

The High Court was asked to rule the laws were invalid under the constitution but that was rejected.

The court has also upheld the actual designation of Papua New Guinea as a processing country, but remaining issues in the case about the decision to remove the man to PNG can be heard in the Federal Circuit Court.

That plan fell short because Malaysia was not a party to the international convention on refugees.

The court found that because Malaysia was not bound by international or domestic law to protect asylum seekers, Australia could not lawfully send those seeking asylum there.

That was because Australia could not guarantee genuine refugees would not be returned to countries where they had a well-founded fear of persecution.

The Manus Island case is different, as Papua New Guinea has ratified the convention.

The Iranian man's legal bid was made on the grounds that two provisions - that the Minister for Immigration can designate another nation a regional processing country, and that "unauthorised maritime arrivals" must be taken to a regional processing centre - are not supported under the Migration Act.

"They would have asked to go home and they can change their view on that up until and including the day," he said.

"That is ... their decision and if they take that decision then we will respect their decision."

Mr Morrison says any asylum seekers trying to reach Australia by boat will still face being turned back under the Government's Operation Sovereign Borders policy.

He says Australia is still helping the people of Iraq.

"We have freed up more than 4,000 places in the Special Humanitarian Program every year - and over this year and the four years of the budget and forward estimates. That is 20,000 places," he said.

"That means that thousands of people whether they are in Syria or Iraq or other places of trouble we are better placed to assist those and particularly those who have contacts and relations with family here in Australia."